Implantable cardiac leads for sensing cardiac signals or delivering pacing or defibrillation impulses to the heart are typically connected to an implantable or external medical device such as a monitor, pacemaker, or cardioverter defibrillator. Cardiac leads may be for temporary or chronic use and provide an electrical pathway between the heart and an associated device. Electrodes located generally at or near the distal end of a lead are coupled to an associated conductor that extends the length of the lead to a connector assembly at the proximal end of the lead that is connected to the associated device.
Conductors commonly used in cardiac leads include a single wire, a mono- or multi-filar coil, drawn brazed, stranded conductors or cabled conductors. Conductors are typically formed from a single conductive metal or alloy material, such as MP35N alloy, or a composite conductive material, such as a silver core wire clad with MP35N alloy. Conductors may be arranged within multiple lumens of a lead body or arranged concentrically within a common lumen.
Conductors are generally joined to lead components, such as an electrode or connector assembly component, by a mechanical joint to form reliable electrical coupling between the conductor and a component. Welding may not be a practical method for joining conductors to lead components due to dissimilar materials that may be present in the conductor. Therefore, a mechanical joint, such as a crimp or a stake is generally preferred. Lead components are often provided with a conductor bore for receiving a conductor. The outer diameter of the bore may then be crimped or staked to cause the inner diameter to be deformed against the conductor and thereby mechanically retain the conductor within the bore and at the same time provide electrical coupling between the conductor and lead component.
Many considerations are taken into account when optimizing the design of a lead. For example, minimizing lead size is important since a smaller lead body is more readily implanted within the cardiac structures or coronary veins of a patient. It is desirable to minimize the lead body diameter by reducing the diameter of conductors carried by the lead. A reduced diameter conductor must be securely connected, both mechanically and electrically, to an electrode or other lead component to ensure proper lead function. Cardiac leads can undergo considerable stresses due to repetitive flexing caused by the motion of the beating heart and forces applied to the lead during an implantation or repositioning procedure. Mechanical joints must reliably withstand these stresses such that breaks in the electrical pathway between the heart and an implanted device do not occur.
Lead components may be provided in standard sizes. For example, lead connector assemblies are generally manufactured according to industry set standards, such as the IS-1 standard, so that leads are compatible with different types of implantable devices. Therefore, a downsized lead body may be fitted with a relatively larger, standard-sized connector assembly. A conductor bore provided on standard-sized components may be too large to be crimped or staked securely around a small diameter conductor. Depending on the machining methods used, the minimum inner diameter of a conductor bore may be limited resulting in a bore that is too large to be effectively crimped around a small diameter conductor.
What is needed therefore, is a conductor assembly and method for mechanically joining a relatively small diameter conductor to a relatively larger-sized conductor bore provided on an associated lead component, which may be a standard sized component. The conductor assembly and method should provide reliable electrical and mechanical coupling of the conductor to an associated component. Furthermore, the assembly and method should allow standard-sized or modular components to be compatible with non-standard or downsized conductors.